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Old 06-17-2020, 11:16 AM   #1 (permalink)
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DARKO Wind Tunnel factoids

1) The DARKO wind tunnel was a 'small', open-return, Eiffel-type, closed test-section design.
2) Drag coefficients derived at ' tunnels with closed test section are generally higher than those from open test sections.' ( Hucho page 433).
3-A) Comparative measurements between wind tunnels of different sizes indicate... that a considerably higher blockage ratio is permissible for automobile aerodynamics. ( Hucho page 404).
3-B) ' Surprisingly, the data of the smallest tunnel, the PININFARINA, are well within the standard deviation of the large tunnels.' ( Hucho page 433)
4) [T]he flow pattern for various vehicles is very similar even at a height of only 2 m above the roadway. Stafford was able to show that with streamlined walls the same result could be obtained with blockage of 20 per cent as in a test section with parallel walls with a blockage of only 5 per cent. ( Hucho page 405)
NOTE: DARKO had steamlined walls and ceiling.
5) Whitfield et al. developed an ' adaptive' wall wind tunnel concept in which the tunnel walls are streamlined such that flow angularity at the walls is the same as for the related streamline in an infinite flow field. This procedure will compensate for blockage ratios up to 30 per cent. And this 30 per cent allows for yaw angle testing up to 10-degrees. ( Hucho, page 406-7 )
NOTE: Wind tunnels with adaptive walls, will compensate for test sections with larger than 30 per cent blockage ratios, for zero-yaw testing.
SECOND NOTE: DARKO was a zero-yaw test facility.
6) ' For climatic tunnels, a nozzle cross-section between 10 and 12 m-squared ( 108 - 129 ft- sq.) is sufficient. The prerequisite is that an empirical blockage factor be determined through calibration measurements in a large wind tunnel or on the road.' ( Hucho page 425)
NOTE : For the 'Spirit of Ecomoder' project, the Toyota Prius calibration model was used, with Cd from the 'large' Toyota wind tunnel, plus the 'small' A2 wind tunnel.
7) Comparability of test data between Toyota, A2, and DARKO is a function of the 'correlation test program' which allowed for the derivation of DARKO's empirical blockage correction factor. ( Hucho page 432)
8) ' When the wheels ( of passenger cars) are integrated into the body the rotation of the wheels appears to have a negligible influence upon the forces and moments acting upon the vehicle.' ( Hucho page 419)
NOTE: rotating wheels make no statistical difference with respect to 'passenger' cars. ( high performance sports cars, SUVs, CUVs, and racing cars are a diferent matter).
9) ' [S]uction of the floor boundary layer has no measurable influence upon the aerodynamic drag of passenger cars.' ( Hucho page 434)
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* The scientific facts are that 'small', closed test section wind tunnels offer no liability with respect to data accuracy as long as protocols for blockage correction or wall geometry enhancements are taken into context.
* With respect to 'Spirit', the numbers reflect that one would experience:
- Cd 0.268 @ DARKO, 2014, as road tested.
- Cd 0.244 if measured at A2 in identical configuration.
- Cd 0.235 if measured at the large Toyota wind tunnel in identical configuration.
* That's all I've ever implied.

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Old 06-17-2020, 01:03 PM   #2 (permalink)
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Needs pix.





No mention of the mesh fence's effect on turbulence on the tunnel walls?
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Old 06-17-2020, 01:32 PM   #3 (permalink)
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mesh and turbulence

Quote:
Originally Posted by freebeard View Post
Needs pix.





No mention of the mesh fence's effect on turbulence on the tunnel walls?
We'd have to talk with Layne and Gary Eaker, ( A2 Wind Tunnel) who consulted on DARKO's design.
The screen could be simply for Foreign Object protection to the honeycomb flow straighteners downstream.
It could also prevent jetsam, if clearing the flow straighteners, ever reaching the 560-hp fan's leading edges.
At DARKO's airspeeds, there's no reason to introduce turbulence since it's full-scale, and supercritical Reynolds number is achieved once to 20-mph air velocity.
The boundary layer buildup on floor,walls,and ceiling, is compensated for with the streamlined walls and ceiling.
Hucho says that any TBL on the floor will not effect drag measurements of a passenger car. I'll take him at his word, having personally run Volkswagen's tunnel for a decade.
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Old 06-17-2020, 02:50 PM   #4 (permalink)
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Quote:
We'd have to talk with Layne and Gary Eaker...
That's what he said as we were walking trough the mesh door. Something about moderating (mangling?) the intake air.

I think speed holes and permeable membranes are under utilized.

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